The objective of the proposed work is to expand our understanding of the mechanism of contractile events as mediated by calcium binding proteins. Specifically, our efforts have been directed toward an understanding of the regulatory mechanism in terms of the detailed structures of the calcium binding proteins. The primary structures of eight homologous calcium binding proteins (MCBP) from fish muscle and a three-dimensional structure to 1.8 Angstrom units resolution of one of these are currently known. We now have evidence that these proteins are closely related to TN-C from mammalian muscle. Fragments of MCBP have been generated which contain each of the two calcium binding sites independent of the other and are being tested for their abilities to bind calcium and interact with tropomyosin. In addition, we are using chemical modification techniques to identify calcium-binding ligands of TN-C, ORD, CD and difference spectroscopy are being used to examine calcium-dependent conformational changes.